Marshall avt275 noise problems !!HELP PLEASE!!

[MARSHALL BUSTER]

Hello guys
Its been a while i hope your all keeping well. Ok heres the story... I bought myself a non working Marshall Mode Four with 4 blown 7293's, so i fixed that and thought to myself i will have a look at the AVT275 again, so i started to check all the same stuff again, i knew everything was working as it had all been checked before more than once, so then i decided to treat it as a new job and start at the beginning. Only this time (and i hear you starting to laugh) i began at the wireing, while tracing the black wires from the main board to the TDA7293 boards the first thing i noticed was that they were each in the wrong conector at CN102/103. I never noticed this the first time round and i knew as soon as i seen it that was the problem all along. I put the amp back together with the wires in the right place, pluged it in and she plays like a dream. Can you belive after all that messing around, the problem could have been solved by simply switching the wires over!!!. So i guess Whats hapend is the person i bought the amp from has put new amp boards in and not put the wires back in the right place, switched it on and all he could hear was the loud buzz/crackle noise that i was getting. Guys thanks a million for all that advice you gave to me, i dont think any of it would help in this situation but im realy glad you were here to help me with the testing side of things. Thankyou very much

I will talk to with you again one day, till then take care.
Mark

[saranden]

Similar trouble

Hi Jazz.
I have been reading this thread re Marshall. I have a AVT150H, and have now come up with a similar problem. To start with, I had replaced the TDA's about 2 years ago...everything was fine until just recently, one of them blew. So I replaced both of them. I have made sure my connections from A and B respectively go to the correct cons con189/230. I sarted the amp up not loaded with speakers and everything is running great, including quiet fans (very rare). However, when I plug a speaker in, very loud BZZZZZZZZ so disconnected it.
I measured the pins on con 1-8 ON con 189 and 230 and have voltages everywhere.
This is what they are. I suspect there is something going to ground that shouldn't be.
Conn 230 (OUTPUT A) Conn 189 (OUTPUT B)
1 0.0 1 0.0
2 -0.7 2 0.1
3 0 3 -47.3
4 5.7 4 5.7
5 0.0 5 0.0
6 -1.5 6 -1.5
7 -30.7 7 -30.4
8 -30.5 8 -30.1

Voltage measurements on CN102/103 are also up the wall with readings of
CN102 CN103
pin 1 47.5 pin 1 47.1
pin 2 47.5 pin 2 -46.7
pin 3 47.2 pin 3 -47.1
All of this running fine, until I complete the circuit with the 8ohm speaker.
What would you suspect is going to ground?

[saranden]

Sorry, I'll make that a bit clearer:
Conn 230 (OUTPUT A)
1 0.0
2 -0.7
3 0
4 5.7
5 0.0
6 -1.5
7 -30.7
8 -30.5

Con 189
1 0.0
2 0.1
3 -47.3
4 5.7
5 0.0
6 -1.5
7 -30.4
8 -30.1

[Jazz P Bass]

[QUOTE=
Voltage measurements on CN102/103 are also up the wall with readings of
CN102 CN103
pin 1 47.5 pin 1 47.1
pin 2 47.5 pin 2 -46.7
pin 3 47.2 pin 3 -47.1
All of this running fine, until I complete the circuit with the 8ohm speaker.
What would you suspect is going to ground?[/QUOTE]

Con 102 & Con 103:
Pin#1: +Vdc
Pin#2: -Vdc
Pin#3: Output pin to speaker: Must be 0Vdc.
Con 102 Pin#2 is the incorrect voltage. Should be -Vdc.
Both Cons have Vdc on the speaker pin. Not good.
I would go back & check the TDA boards for solder bridges or opens.

[saranden]

I detached the TDA boards and read the voltages which were true at pins 1-8 and pins 1-3. I checked my traces etc on the TDA PCB's and they are all good. I then got out my clip on loupe magnifier and was shocked to see TDA7294V and not TDA7293V. I bought 10 of these from Thailand as TDA7293v's at £1 each including free postage so I thought this was a bargain. I will contact the seller and hopefully get some form of compensory payment or NOT. I have purchased some more from a UK seller at £5 each which is a normal price...and I will refit them and confirm that it is all OK as I expected it to be. I have installed IC sockets on my TDA board for quick release, and installed a plastic rail below the boards to stop the TDA PCB's being shocked off the component and heat sink. I bet Marshall wouldn't consider doing this...the PCB cost £50 here in the UK. Easy math £10 versus £100...I could make a fortune converting these babies. I'll post another update when I have received and installed the new ones. Thanks again

[Jazz P Bass]

The TDA 7294 does not use Pins 5, 11 & 12.
Pin 5 is clip detect. Pin 11 is boot driver & pin 12 is boot loader on the 7293.
The bootstrap capacitor should go from Pin 6 to Pin 14 on the 7294.
Other than wattage, that is the only difference that I can see.

[Jazz P Bass]

The TDA 7294 does not use Pins 5, 11 & 12.
On the 7293 Pin 5 is clip detect. Pin 11 is buffer driver & pin 12 is bootstrap loader.
The bootstrap capacitor should go from Pin 6 to Pin 14 on the 7294.
On the AVT I believe the bootstrap cap goes from pin 6 to pin 12.
Other than wattage, that is the only difference that I can see.

[Jazz P Bass]

The TDA 7294 does not use Pins 5, 11 & 12.
On the 7293 Pin 5 is clip detect. Pin 11 is buffer driver & pin 12 is bootstrap loader.
The bootstrap capacitor should go from Pin 6 to Pin 14 on the 7294.
On the AVT I believe the bootstrap cap goes from pin 6 to pin 12.
Other than wattage, that is the only difference that I can see.

[Jazz P Bass]

TDA9293 vs TDA7294

The TDA 7294 does not use Pins 5, 11 & 12.
On the 7293 Pin 5 is clip detect. Pin 11 is buffer driver & pin 12 is bootstrap loader.
The bootstrap capacitor should go from Pin 6 to Pin 14 on the 7294.
On the AVT I believe the bootstrap cap goes from pin 6 to pin 12.
Other than wattage, that is the only difference that I can see.

[Jazz P Bass]

TDA9293 vs TDA7294

The TDA 7294 does not use Pins 5, 11 & 12.
On the 7293 Pin 5 is clip detect. Pin 11 is buffer driver & pin 12 is bootstrap loader.
The bootstrap capacitor should go from Pin 6 to Pin 14 on the 7294.
On the AVT I believe the bootstrap cap goes from pin 6 to pin 12.
Other than wattage, that is the only difference that I can see.

[saranden]

TDA replaced

Hi Jazz...I replaced the TDA's with TDA7293, although I did try the pin rearrangement on the TDA7294 but had no luck with the voltages...I was still sending +voltage to pin# 3 from pin# 14 of TDA to pin# 8 on TDA PCB. I would be interested to see a schematic of how to rewire the mini PCB to accept the 7294V. As it was..with the TDA7294, I was connecting Bootstrap Loader to OUT.
Thumbnails below of IC socket and plastic rail to support. On heat sink just for photo!!

[saranden]

Thanks

Oh Jazz, I meant to say...she runs perfect with the TDA7293V as expected. It is a very interesting thread, for which I will make a donation and grateful thanks.

[Jazz P Bass]

Really glad to hear that the amp is operational.
The thread from hell all because of a bogus, crap chip.